Diffusion of helium has been characterized in natural zircon. Polished slabs of zircon oriented either normal or
parallel to c were implanted with 100 keV 3He at a dose of 5x10153He/cm2. Implanted
zircons were annealed in Pt capsules in 1-atm furnaces for times ranging from 20 minutes to 6 weeks at
temperatures from 298-604 C. 3He distributions in the zircons were measured with Nuclear Reaction
Analysis using the reaction 3He(d,p)4He. For diffusion normal to c we obtain the following Arrhenius
relation: D = 2.3x10-7 exp(-146 ± 11 kJ mol-1/RT) m2sec-1. For diffusion parallel to c,
we obtain: D = 1.7x10-5 exp(-148 ± 17 kJ mol-1/RT) m2sec-1. Although activation
energies for diffusion normal and parallel to c are comparable, there is marked diffusional anisotropy, with
diffusion parallel to c nearly 2 orders of magnitude faster than transport normal to c. These diffusivities bracket
the range of values determined for He in zircon in the bulk-release experiments of Reiners et al. (2004),
although the role of anisotropy could not be directly evaluated in that study. Since the diffusion of He in zircon
exhibits such pronounced anisotropy, helium diffusional loss and closure cannot be modeled with simple
spherical geometries and the assumption of isotropic diffusion. A finite-element code (CYLMOD) has recently
been created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients.
We will present some applications of the code in evaluating helium lost from zircon grains as a function of
grain size and length to diameter ratios, and changes in radial and axial He concentration gradients for various
thermal histories. PW Reiners, TL Spell, S Nicolescu, KA Zanetti (2004) GCA 68, 1857-1887

GA31B-02

From Late Ordovician-Silurian Arc Magmatism to Devonian Terrane Amalgamation and Juxtaposition of Baltican and Laurentian Crust: New Evidence from Liverpool Land, East Greenland Caledonides

The Liverpool Land basement high, East Greenland, is composed of two Caledonian terranes with vastly
different tectonomagmatic and-metamorphic histories. The two terranes are separated by a N-dipping
composite shear zone, composed of the Gubbedalen Shear Zone, and a younger brittle fault, Gubbedalen
Extensional Detachment Fault. The upper, northern plate is dominated by an Ordovician-Silurian plutonic
complex intruding partly migmatized metasediments. Two magmatic phases of the Hurry Inlet Composite
Pluton provide U-Pb ID-TIMS ages of 445 and 438 Ma, and the Hodal-Storfjord Monzodiorite an age of 424 Ma.
Petrographic and geochemical data indicate that these granitoids represent a continental magmatic arc,
formed above a west-dipping subduction zone in the Iapetus Ocean. By contrast, the lower, southern plate
comprises an eclogite- and garnet peridotite-bearing terrane, the Liverpool Land Eclogite Terrane (LLET).
Polycrystalline quartz inclusions surrounded by radial cracks in garnet in several eclogite samples, indicate
that the eclogites were exposed to UHP-conditions. U-Pb data from an eclogite lens indicate protolith formation
no older than 1628 Ma and eclogite facies metamorphism at 400 Ma. Data from a migmatized orthogneiss
indicate protolith formation at 1640 Ma and a metamorphic overprint at 383 Ma coinciding with the time of
crystallization of extensive anatectic magmas at c. 385 Ma. The evidence for Mesoproterozoic protoliths and
Devonian HP-metamorphism, and the occurrence of garnet peridotites, distinguish the LLET from other
terranes in East Greenland. Also notable is the absence of Ordovician-Silurian plutons in the lower plate and of
Mid Devonian overprints in the upper plate of Liverpool Land. These observations indicate that LLET has an
exotic origin with respect to Laurentia, demonstrating instead an affinity to the Western Gneiss Region in
Norway. Paleogeographic reconstructions show that the Western Gneiss Region and Liverpool Land were
adjacent to each other in Mid Devonian time (e. g. Dewey and Strachan, 2003). We thus propose a Baltic origin
for the LLET. Its amalgamation to the Laurentian plate must have occurred shortly after (U)HP-metamorphism,
temporally close to the time of thrusting of the Caledonian allochthon in East Greenland. Liverpool Land
represents, therefore, a key area to understand the timing of closure of the Iapetus Ocean and the late stages
of collision between Baltica and Laurentia.
Reference:
Dewey, J. F., and R. A. Strachan (2003), Changing Silurian-Devonian relative plate motion in the Caledonides:
sinistral transpression to sinistral transtension, Journal of the Geological Society, 160, 219-229

The Barry gold deposit is located in the Urban-Barry Archean greenstone belt in the Abitibi subprovince of
Québec. Mineralization is associated with contemporaneous albite-carbonate-quartz veins that are straight
N064/64 and folded N020/60 and their surrounding mafic volcanic rocks altered to carbonate-quartz-pyrite and
locally to biotite-carbonate. The auriferous zones are spatially associated with NE-trending shear zones with
moderate southerly dip. Volcanic units strike 055-060 and dip 40° SE, but the ore envelope (>2 g/t Au)
is constrained from surface to a depth of 30m in an antiformal shape and does not favour a specific volcanic
unit or facies. Free gold is found in albite-carbonate-quartz veins, syn-ore altered host rocks, and locally within
quartz veins cutting quartz-feldspar porphyry (QFP) dikes. The deposit has gold resources indicated at 52,300
oz (385,000 mt at 4.23 g/t Au) and inferred at 126,600 oz (966,000 mt at 4.07 g/t Au). Pre-ore chlorite-biotite-
magnetite-ilmenite-rutile alteration of mafic volcanics makes it difficult to determine the composition of their
protoliths. Discrimination diagrams were used to categorize the mafic volcanics into three types: Type A being
consistent with mid- ocean ridge basalt (MORB), while Types B and C are consistent with basalts transitional
from tholeiitic to calc-alkali. Mafic volcanics are cut by pre-ore diorite, pre- and post-ore QFP, and quartz
monzonite dikes and plugs. Mafic and intrusive units have also undergone a post-ore biotite-chlorite-
carbonate-muscovite-epidote alteration. In mineralized zones albite-carbonate-quartz veins comprise 5-15%
volume of the mafic volcanic package, locally pinch and swell or are boudinaged, and are 1 to 5cm wide. They
are composed of albite (20-50%), carbonate (30-40%), quartz (20-40%), and trace biotite ± sericite,
chlorite, pyrite, magnetite, and fine-grained visible gold. The associated syn-ore carbonate-quartz-pyrite
alteration comprises carbonate (ferroan dolomite to ankerite: 5-45%), quartz (2-8%), pyrite (2-8%), pyrrhotite
(trace-3%), trace chalcopyrite, and native gold. The biotite-carbonate alteration is associated with veins in
areas of strong foliation and shear zones, and is composed of biotite (10-55%), carbonate (20-30%), pyrite
(trace-2%), and rare pyrrhotite. Gold is present as inclusions in, filling cracks in, and adjacent to pyrite grains.
Gold alloys contain variable amounts of Ag (3.6-9.04 wt%), with other elements near or below detection limit.
Zoned pyrite contains variable amounts of Ni, Co, W, Ti, Te; trace Se, Hg, and Zn; and locally trace amounts of
Au and As. The timing of mineralization is well constrained by U-Pb zircon dating of pre-mineralization diorite
and post-mineralization QFP dikes (containing xenoliths of mafic volcanic rock with pervasive syn-ore
carbonate-quartz-pyrite alteration). Analyses of single zircon grains give concordant and overlapping data with
indistinguishable ages, yielding an average age of 2697 ± 1Ma for gold mineralization. To our knowledge,
this is the most precise age yet established for Archean gold mineralization. It shows that gold mineralization
was coeval with regional folding and faulting, as well as arc-related, syn-collisional intermediate to felsic
magmatism.

GA31B-04

Deformation and Recrystallization of Zircon and its Influence on the Isotope Systems: a Case From a Shear Zone in Anorthosite of the Lindas Nappe

The chemical robustness of the common accessory mineral zircon (ZrSiO4) under the range of conditions
presented in the earth's crust, and its ability to embed U, has led to its widespread use in petrology, especially
in the field of geochronology. Nevertheless, it is also well known that there can be element mobility in zircon,
most importantly Pb-loss related to hydrothermal activity and weathering. A recent discussion concerns the role
of crystal-plastic deformation and microstructures in zircons and their effect on zircon geochemistry. In this
study we have investigated an example of a deformed and partly re-crystallized zircon. The sample stems from
a 1-2 cm-wide shear zone within a granulite facies rock of anorthositic composition located in the Bergen Arcs,
an arcuate structure composed of Caledonian thrust sheets. No zircon could be found in the anorthosite
besides the shear zone, but exceptionally abundant zircon grains of large size occur in the deformation zone.
The grains are subrounded and anhedral. They are locally fractured and crystallographically bent, and in part
are surrounded by domains of small polycrystalline zircon. Trails of the smaller (new- or recrystallized) zircons
locally extend along the pressure shadow of larger fragments and grains. Detailed crystallographic orientation
analysis using Electron Backscatter Diffraction (EBSD) reveals that large grains are severely deformed, both by
fractures and crystal plastic deformation. The latter is apparent throughout the whole grain but is most
pronounced at grain edges exhibiting change in crystallographic orientation of 5° /100 μm. Sub-
grain boundaries with mis-orientation of more than 2° are only seen close to grain edges. Re-crystallized
grains are strongly crystal plastically deformed, show abundant sub-grain boundaries and changes in
orientation of up to 30°/100 μm. Analyses by ID-TIMS U-Pb show that the original zircon is extremely
low in U at 2-5 ppm but higher values of over 16 ppm are found in the newly nucleated grains. The results for
the original grains are variously discordant and define a discordia line with an upper intercept of 905 ± 17
Ma. This age is younger than the age of intrusion of the anorthosite, which must predate the time of high-grade
metamorphism at 930 Ma. Therefore it is interpreted to indicate an event of metasomatism or magmatism that
created the vein introducing, among others, 6000 ppm Zr. This vein was then affected by intense shearing
during the Caledonian reworking of the Precambrian granulites. The lower intercept of the discordia line is
constrained at 425 Ma by the analyses of the small anhedral grains. Spot U-Pb analyses by LA-ICPMS
demonstrate that severe discordance is also found in small domains within the large grains. It reveals as well
discordance between the small re-crystallized zircons, where significant crystal distortions can be seen on the
EBSD maps. Because of the low U content it is not possible to explain the extensive Caledonian Pb loss of the
original grains by metamictization. This requires instead a mechanism of enhanced diffusion of Pb or leaching
of Pb by fluids from high-strain zones, or expulsion of Pb during local re-crystallization. A complete re-
equilibration would have been facilitated by their very high sub-grain boundary/volume and crystal
bending/volume ratio as well as easy access through fine grained matrix network.

GA31B-05

High-Precision ID-TIMS U-Pb Zircon Geochronology of Two Transcontinental Cenomanian Bentonites From the Western Canada Foreland Basin

A detailed allostratigraphic framework exists for Upper Cretaceous rocks over a large part of the Western
Canada foreland basin in Alberta and British Columbia. Sediments are dominated by offshore marine
mudstones, and more sand-rich facies of nearshore and coastal plain environments. Biostratigraphically-
useful molluscan fauna are sparse, and biostratigraphy has largely been based on long-ranging benthic
foraminiferal assemblages. In Canada, a number of extensive bentonites have been mapped in Cenomanian
strata and traced into the United States portion of the basin, as far south as New Mexico, where the
relationships to the ammonite zones can be determined. Two of these continent-scale bentonites provide
important calibration points for the Upper Cretaceous timescale. The 'X-bentonite' lies within the Middle
Cenomanian Acanthoceras amphibolum ammonite zone and the 'B Bentonite' (United States) or 'Bighorn
River Bentonite' (Canada) lies within the Late Cenomanian Neocardioceras juddi ammonite zone. The
Bighorn River Bentonite lies a few meters below the Cenomanian-Turonian boundary and on a regional scale,
provides an approximation for that boundary. A sample of each of the X and Bighorn River bentonites was
collected from the Canadian Alberta Foothills and zircons were separated using ultra-low contamination
methods. Single zircon crystals were chemically-abraded and dated by high-precision U-Pb ID-TIMS methods
utilizing the ET535 tracer solution (www.earth-time.org). The agreement of multiple analyses within
analytical uncertainty indicates an absence of much older xenocrystic cores. The weighted-mean
206Pb/238U ages we have measured for the two samples are 95.87 ± 0.10 Ma for the X-
bentonite and 94.29 ± 0.13 Ma for the Bighorn River Bentonite (uncertainties at 2 σ level of
confidence). These ages are approximately 1 m.y. older than previously published 40Ar/39Ar ages
for these two bentonites (94.96 ± 0.5 Ma and 93.3 ± 0.2 Ma respectively), and are more precise. Our
zircon age for the Bighorn River Bentonite establishes a more precise maximum age for the Cenomanian-
Turonian boundary of 94.29 ± 0.13 Ma. These new ages from the Western Canada Foreland Basin not
only provide a more precise global framework for Cretaceous geochronology, but also allow more accurate
measurements of timing and rates of tectonic and eustatic activity in Western North America.

The 2.020 Ga Vredefort impact basin is cored by Archean crystalline basement centered at 27° S, 27°
30'E, 120 km southwest of Johannesburg, South Africa. The originally ∼250 km diameter impact basin is
presently the largest and oldest recognized impact structure, and numerical modeling predicts that an event of
this magnitude would have produced a melt sheet several kilometers thick. The Vredefort melt sheet is not
believed to have survived erosion with the possible exception of a m-scale body of foliated metanorite in the
crystalline central uplift. In this study, the morphology and shock microstructures of a large number of zircons in
the foliated norite body and adjacent Inlandsee leucogranofels orthogneiss unit have been analysed and
compared as a first step to determining the metanorite protolith. Zircon microstructures were imaged on
external and fractured surfaces by SEM using secondary electron (SE), and backscatter electron (BSE)
detectors. The SEM mounting stub and zircons were then cast in an epoxy plug and polished to reveal internal
zoning structures using SEM colour cathodoluminesence (SEM-CL). The whole grain and internal images
were then grouped based on internal and external characteristics in order to study their overgrowth patterns
and crystallization history. The planar features (PF's) in the leucogranofels orthogneiss and most of the foliated
metanorite zircons were found to be discontinuous and sometimes truncated by overgrowth of unshocked
zircon. This differs from the PF's seen in the zircons from outside the central uplift, where they are sharp,
pervasive and continuous throughout the grains. Of the 87 grains analysed from the leucogranofels
orthogneiss, all exhibited shock features and 22% of these exhibit post-shock zircon growth or recrytallization.
Of the 45 grains analysed from the foliated metanorite, a larger proportion, 32%, are recrystallized, shocked
grains. Unique zircon recrystallization features have been observed in the metanorite population including
internal replacement or recrystallization of shocked and annealed Archean domains by post-impact growth.
The results confirm that the metanorite sample is the only one to contain a component (13%) of unshocked,
euhedral zircons (which have a syn-impact age of 2019 ± 2 Ma)whereas the granofels zircons are all
shocked and show a range of shock deformation states, allowing for an impact melt origin for the metanorite
protolith. Having documented different degrees of shock metamorphism, the zircons from the two rock units
are now ready for detailed microbeam isotopic analysis to test the impact melt origin the metanorite parent
magma.